Numerous methods have been proposed to solve this problem, such as mechanical scraping or heating of the walls. These methods are costly and cannot always be applied.
In order to improve the rheology of crude petroleum oils, SHELL did the pioneering work with FR 1,575,984: it teaches that macromolecular compounds of the "comb" type, constructed on the model of a principal hydrocarbon chain on which fairly long lateral hydrocarbon chains are grafted, that is, chains with at least 14 carbon atoms and at most 30 carbon atoms, can perturb the crystallization of heavy paraffins. This property is manifested well by macromolecules, which have an average molecular weight (number-average molecular weight M.sub.n, the definition of which is recalled here: EQU M.sub.n =.SIGMA..sub.i NiMi/.SIGMA..sub.i Ni,
where Mi are the molecular weights of the individual species Ni present in the polymer) is between 1000 and 1,000,000 and preferably between 4000 and 100,000. Thus, the utilization of additives was suggested, most frequently polymeric additives, the role of which is to retard or to modify the crystallization of paraffins and thus improve the flow properties of the oil and prevent agglomeration of the crystals formed on the walls.
Numerous studies then attempted to improve the efficacy of these first additives of polymeric nature, either by the synthesis or by the formulation, in order to adapt them to different types of crude oils encountered and to ameliorate successively the difficulties of synthesis and/or handling of the various generations of products, for example, among the most effective ones, C.sub.18-30 acrylate copolymers, preferably mainly C.sub.20-22 ones, with a heterocyclic monomer, notably vinylpyridine [U.S. Pat. Nos. 2,839,512 (1958) and FR 2,128,589 (1972) of SHELL]. The presence of polar units confers a dispersing character to the polymer, which permits avoidance of deposition of paraffins on the wall. Now, because of the higher reactivity of long-chain acrylates in comparison to the polar comonomers, incorporation of the latter is generally very difficult and the dispersing effect related to the rate of incorporation of the polar comonomer frequently remains very low.
In spite of these successive improvements, these additives cannot be applied universally to all crude oils, each one representing a particular case and presenting its own problems.